When using the great and mighty ethanol gas (don't get me started) and especially when this gasoline sits for a long time, it gums up your jets, gas lines and well in the generator (remember the same thing happens to your car).
The easiest way to prevent much of this "gumming up" is to use non-ethanol gas. Seeing as how 99.9% of stations don't carry it, this is next to impossible. It was recommended to me, when ever you have to go long periods of time without running your generator (or car), use an ethanol treatment in your tank. I am using a product called Stabil and it seems to work. This treatment will help keep the lines clear. It is important to note, only use this treatment when really needed because overuse will result in premature deterioration of your gas lines.